1. Field of the Invention
The present invention relates to the arrangement of a capacitor which is utilized in a tank type circuit breaker.
2. Discussion of Background
In general, such capacitor is arranged between the contacts to perform voltage-dividing so as to relatively equalize the voltages at each breaks in a multi-interrupting point type circuit breaker or improves the breaker performance under severe interrupting conditions wherein a high rate of rise of restriking voltage is caused in case of a short line fault and so on. The voltage-dividing capacitor in the multi-break circuit breaker can also contribute to the improvement of the breaker performance. In addition, in case of the single break circuit breaker, a capacitor between contacts is usually used when a great interrupting capacity in a short line fault is required.
The capacitor is generally arranged within a grounded tank to surround an arc-extinguishing chamber therein. Several proposals have been made on the assumption that the capacitor is arranged in the grounded tank (see Japanese Examined Patent Publication No. 18618/1976, Japanese Unexamined Utility Model Publication No. 38669/1978). FIGS. 3 and 4 are a front view and a vertical sectional view showing the structure of a conventional tank type circuit breaker as disclosed in e.g. the Japanese Unexamined Utility Model Publication No. 38669/1978. In FIGS. 3 and 4, a grounded tank 2 is shown supported on a platform 1. The grounded tank 2 has a pair of bushings 3, 3 mounted on the top so as to project upward. Conductors 4, 4 extend through both bushings 3, 3. Bushing current transformers 5, 5 are arranged between the grounded tank 2 and each bushing 3 so as to be insulated from and be wound around each conductor 4, thereby detecting a current through each conductor. The grounded tank 2 includes an insulation member 6 which is attached on one of the end walls of the tank. A piston 7 is attached to the insulation member 6, has a through hole and is connected to one of the conductors 4. Into the piston 7, is fitted a puffer cylinder 8 so as to be slidable with respect to the piston. A movable contact 9 is inserted into the through hole of the piston 7 so as to be slidable therein, and is fixed to the puffer cylinder 8. A nozzle 10 is fixed to the puffer cylinder 8 so as to surround the movable contact 9 with a predetermined gap. The piston 7 also has an insulation member fixed thereto. A stationary contact 12 is fixed to the insulation member 11 so that it is opposite to the movable contact 9 so as to be engageable therewith and disengageable therefrom. The stationary contact is connected to the other conductor 4. A capacitor 13 is arranged between the piston 7 and the stationary contact 12 in the neighborhood of both contacts. The capacitor has a structure wherein a plurality of ceramic elements having a high dielectric constant are arranged in series and parallel combinations. The piston 7, the puffer cylinder 8, the movable contact 9, the nozzle 10, the insulation member 11, the stationary contact 12 and the capacitor 13 form an arc-extinguishing chamber 14 as an interrupting unit. The grounded tank 2 is filled with SF.sub.6 gas 17 as an insulation medium. Operating means 15 as shown in FIG. 3, is provided to reciprocate the movable contact 9 so as to engage with and disengage from the stationary contact 12. Each conductor 4 has a main circuit terminal 16 attached to the leading edge 4a.
The interrupting operation is as follows: Current flows from the conductor 4 in one of the bushings 3 to the conductor 4 in the other bushing 3 through the contacts 9, 12 being connected in the arc-extinguishing chamber 14. The bushing current transformers 5, 5 detect the currents flowing through the conductors 4. When one of the current transformers detects a ground fault current, the operating means 15 activates to move the puffer cylinder 8, the movable contact 9 and the nozzle 10 in the right direction indicated by an arrow A in FIG. 4 so as to disengage the movable contact 9 from the stationary contact 12. At that time, a high voltage of arc is created between the contacts 9 and 12. Since the arc has a great deal of energy, it gives radiant heat around itself, and it raises the pressure and temperature of the SF.sub.6 gas 17 to high levels. The SF.sub.6 gas 17 in the puffer cylinder 8 is compressed between the fixed piston 7 and the moved puffer cylinder 8, so that the compressed SF.sub.6 gas 17 is blasted to the arc through the nozzle 10 so as to cool it, thereby interrupting the current.
In case of an a.c. circuit breaker, when the current comes to 0, it is interrupted, and then a recovering voltage is applied between the contacts of the interrupting means. The circuit breaker is very sensitive to the recovering voltage. The initial rate of rise of the recovering voltage has a significant effect on the breaker performance. The waveform of the recovering voltage at that time is determined by inductance, capacitance and several kinds of loss components, which distribute in the line. When a ground fault happens to the line, the capacitor 13 inserted between the line and ground can be delay the rise of the recovering voltage on the line and reduce severity in the interrupting conditions. For the reasons, in circuit breakers for a high-amperage current which is required for severe interrupting performance at the time of a short line fault, it is well known that there is provided such capacitor.
Since the conventional tank type circuit breaker is constructed as above, the capacitor 13 is exposed to the insulation medium 17 having a high temperature or the arc radient heat, which accelerates the deterioration of the capacitor 13. In addition, since there is some space required between both contacts 9 and 12 and the capacitor 13, and insulation distance is needed between the capacitor 13 and the grounded tank 2, a large diameter of grounded tank 2 is required, which is disadvantageous.